The present invention relates to compositions and a method for the hydraulic fracturing of subterranean formations penetrated by a well. Particularly, the present invention relates to the use as hydraulic fracturing fluids of acids, especially dilute acid solutions thickened with copolymers of dimethyl diallyl ammonium chloride and a monomer capable of postreaction after polymerization to provide branching or partial crosslinking which extends the chain length of dimethyl diallyl ammonium chloride polymers. Suitable monomers capable of postreaction after polymerization to provide branching or partial crosslinking include N-methylolacrylamide and diacetone acrylamide.
In the art of producing oil or gas from a subterranean formation, it is well known that production can be greatly increased by hydraulically fracturing the formation. In a hydraulic fracturing operation, a fracturing fluid is forced down a well bore under high pressures to fracture the rock formation surrounding the well bore. The pressure is then released allowing the oil or gas to seep through the fractures into the well bore where it is then pumped to the surface.
Most commonly, fresh water or aqueous brine are used as the fracturing medium in oil field operations. The brines usually contain from a few parts per million to high percentages of dissolved salts. In fact, oil field brines sometimes contain total dissolved solids of up to about 10 percent or higher. Particularly, common brines which are difficult to use with polymeric additives are those having dissolved alkaline earth metal salt concentrations of more than 1,000 rpm. Most oil field brines contain at least several hundred ppm of calcium in addition to 2 percent or more sodium chloride and potassium chloride.
In addition, it is also well known in the art of producing oil or gas from a subterranean formation that fluid production can occasionally be stimulated by injecting acid solutions into the formation by way of the well bore. This is especially true when the reservoir contains large amounts of carbonate rock such as limestone, dolomite, and the like. The acid reacts with the carbonate content of the rock, thereby creating channels in the rock between the reservoir and the well bore. This increases the effective drainage area of the well bore, stimulating production.
The most commonly used acid for this purpose is hydrochloric. However, other acids such as hydrofluoric, nitric, formic, acetic and sulfonic have also been highly successful in increasing production in calcareous formations. Mixtures of two or more different acids have also been used, especially mixtures containing hydrofluoric acid. The acids are usually employed as 1 percent to 35 percent by weight solutions in water. However, because of the scarcity of fresh water and economics, it is often necessary to employ oil field brine as the aqueous medium in place of water. In these cases, the aqueous medium will be an acidic brine.
Acidizing and hydraulic fracturing are generally combined into one treatment by employing an acidic fluid under hydraulic fracturing pressures. This combination treatment of the well is called acid fracturing and it stimulates production by obtaining the benefits of both the chemically created channels and the pressure created fractures. When acid fracturing, the aqueous fluid may be an acid solution or an acidic brine as described above.
It is desired that the fracturing fluid be viscous in nature since this will permit the fracturing fluid to remain in the fracture long enough to permit build-up and maintenance of sufficient pressure to open a fracture. Moreover, a viscous fracturing fluid is capable of supporting propping agents, which are particulated solids suspended in the fracturing fluid for the purpose of maintaining the fracture in an open condition, by being deposited in the fracture once created. Generally, it is desired that the fracturing fluid have a viscosity of at least about 25 centipoises, as measured by Fann Model 35 Viscometer at 25.degree. C. and at 100 rpm, for a 1 percent by weight concentration of thickening agent.